for levelling a lathe?
|461 forum posts|
In the latest MEW there is an article about using a digital level for leveling machine tools. Now these levels have a resolution of 0.1 deg. What the accuracy of that number is, nobody knows. If my maths are not completely off, 0.1 deg amounts to a distance of 1.7 mm in one meter (could do it in inches too if it must be!)
I own a bubble level with a sensitivity of 0.05 mm per meter, and this means between two lines about 5-6 mm apart on the vial. this is more than 30 times better (and it is possible to see much smaller deviations due to the aforementioned scale distance).
So does the accumulated wisdom here think that leveling to 0.1 deg is 'good enough'?
Strange, to be sure I looked at the title in MEW, there it was 'levelling', but the spill chucker in this entry field wants 'leveling'. Who's right?
|3334 forum posts|
|I would stick with the bubble level, as long as there is no twist in the bed, the absolute dead level is not necessary.|
|John McNamara||19/12/2011 12:10:37|
1313 forum posts
No way is .01 deg enough accuracy to eliminate the effect of twist of the bed, To me that is the bugbear. A lathe will work fine out of level as long as the bed is not twisted. Although best practice is to keep it properly level in both planes...
A very useful paper here....Link
I used the part on mapping my headstock alignment to the bed. Take 30 dial indicator measurements, The indicator is mounted on the saddle; using a 300mm test bar at the 180 deg Max and Min points. Then graph the results in excel it is very accurate.
The bit about putting paper shims under the feet? I assume they are replaced by metal shims!
And if your lathe stands on a concrete floor beware of the weather! This week I put a part on the lathe and started turning....The part was a 200mm round Eclipse magnetic chuck that had been abused and needed refacing. it was a little out of balance. the job was for a neighbour who rescued it, and good friend. his lathe was a little small for the job. Normally this would not be a problem. However the bed was vibrating slightly, I found that the levelling jack screws under the centre leg were clear of the floor (the lathe 2 metres between centres is supported at 3 places with 8 jacks a bit of a pain to set up). It is amazing how resonance can show up I must have been near the critical speed. With bigger machines it is important to check the level very regularly. Melbourne where I live has reactive clay subsoil A bit of rain and it can move, after a re-level the vibration problem was gone.
Edited By John McNamara on 19/12/2011 12:34:47
Edited By John McNamara on 19/12/2011 12:36:15
|Peter G. Shaw||19/12/2011 13:24:51|
1232 forum posts
Tubal Cain aka Tom Walshaw gives a method of lathe bed adjustment by turning tests at the end of his book, Workholding in the Lathe, WSP15. He says that the turning test is the only way that a lathe can be correctly set up. He also gives a design for jacking screws thus eliminating any need for shims, paper or otherwise.
Peter G. Shaw
|903 forum posts|
So does the accumulated wisdom here think that leveling to 0.1 deg is 'good enough'?
Not by a long way, if you want to do the job properly. A precision level with 0.05mm/Metre sensitivity or better - at my previous employment we had a 0.02mm/Metre block level as well as 0.05 box levels - & aim for 1/2 a division error or better. I was able to "borrow" overnight the 0.02 level to install my Myford & it levelled to within the width of a graduation line in both planes - near enough !. When checked again a couple of years afterwards there had been very little movement & only minor adjustments were required.
the absolute dead level is not necessary.
It is if you want to check alignment to British or other National Standards. The first check on all those I have worked with is to check absolute level in both planes & there is a (tight) tolerance specified. Some machines (slant bed lathes for example) require a levelling fixture to hold the level, due to lack of suitable features on the bed. One of the "challenges" of installing machines & demonstrating the alignments is that you are very limited in what you can tweak - due in part to the levels being "tied up". It was frequently necessary to push one check (say, levels)to one side of the tolerance band in order to get another check (say, squareness) to come in on the other side. Small adjustments & frequent re-checking of all the values being the order of the day.
|Clive Hartland||19/12/2011 14:14:44|
2630 forum posts
0.1 deg is 6minutes which is a big error to me.
I am used to working to within 1 or 2 Sec.
Bear in mind that the tolerances give a big margin for error on bubbles with a graduated scale.
If you set up a lathe bed then you have to, 'End for End' the bubble.
This means you level in one position and then turn the plate level 180deg. and take half the error shown out of the bubble adjustment and half out of the bed adjustment.
You have to do this slowly to allow the bubble to settle and do not touch the bubble or mount with your fingers and do not breathe on the bubble, if you do the bubble will run and give a false reading.
'Never trust a bubble', Always check and then re-check.
After this sequence you can safely continue the check of the bed level.
The bubble should indicate its tolerance. ie. 20" or such, again bear in mind you are leveling a bed and the tol. is not required for that purpose.
For accuracy then the best method is Auto Collimation where the bed is checked every 300 mm and a graph drawn showing deviation of the bed.
This requires a theodolite with an auto-collimation eyepiece and a surface silvered mirror in an adjustable mount which is traveled along the bed and readings taken.
Readings are twice the error. Incident ray/reflected ray is twice the angle.
To me this would be the only method on a long bed machine.
1935 forum posts
When I worked in a precision toolroom the answer to your question would be an emphatic no. However I now make relatively small model engines etc for my own interest and enjoyment. In this pursuit I turn relatively short lengths and machine small surfaces, so extreme accuracy over the whole bed of a lathe for example is not really necessary for me.
So nowadays, that sort of accuracy (0.1 degree) suffices for my simple needs. It is very easy to get anal about extreme accuracy in a home workshop, I'm not producing parts for NASA or even Cosworth. As long as I can turn to within .01mm reasonably parallel over 75 to 100mm, I'm happy. So the answer to your question is both yes and no, it all depends on what you are trying to achieve and your own ambitions,
For twist in a lathe bed I usually use 'Rollie's Dad's Method' to check it and it works ok for my purposes.
Best regards and all the best wishes of the season, Happy Yuletide and New Year,
|Harold Hall 1||19/12/2011 19:36:07|
|418 forum posts|
The only reason for setting a lathe bed perfectly level at both ends is to ensure that it is not twisted but this would also be true if the lathe sloped by exactly one degree at both ends, in this case though, how do you check a one degree slope? However, lathes are made to given tolerances and even if the bed is not twisted the lathes mandrel may not be perfectly in line with the bed and a little twist may be worthwhile adding to minimise the error.
My recommendation is therefore, set the lathe level using the most accurate level available, probably a DIY version. As most home workshops do not have a precision level this will most often be the case.
Having done that, turn a test piece, checking how parallel this is, and from the result adjust the lathes mounting to minimise the error. Repeat the test as necessary until a satisfactory result is achieved.
Having achieved a satisfactory result, now turn a test piece between centres, measure along its length and this time adjust the tailstock so as to reduce the error. Again, repeat the test as necessary.
Have a look here for more guidance.
Incidentally, I have a precision level and it never gets used. I set the lathe using the above tests, but if eventually I turn an important component and the result has some taper I readjust the lathe's mounting, if possible, using the component as my test piece.
I agree with Peter and Terry, by the way, what is "Rollie's Dad's Method"? (I have now googled it and come up with the answer)
Edited By Harold Hall 1 on 19/12/2011 19:39:13
167 forum posts
To answer your other question:
Levelling is English, leveling is American. The poor dears can't afford the extra ink
As to this much accuracy, well Drummonds stand on one foot so it is not an issue.
Seasons Greetings to you all
|461 forum posts|
Thanks Gents, I am quite happy that I agree with you. So for 'ordinary work' the alignment is not so important, but to do it according to the text books a digital level is not sensitive enough.
I want to make it clear that I'm not trying to level my lathe(s). One has no leadscrew, so it is only possible to turn as far as the topslide reaches. And the other one sits on a very massive cabinet and was most probably never removed from there since leaving the factory. But - both are levelled (with the sensitive vial level) via adjustable foots on the floor level.
Now I would hope that one of the DC's would put an explication in the next issue, as it was done about the Magnesium steel in 184.
Ah yes, leveling and levelling - separated by a common language they say...
|Douglas Johnston||20/12/2011 09:20:12|
716 forum posts
The MEW article seemed to imply that the digital level came with a bubble vial on top and it was not clear to me what was going on between the digital reading and the vial reading. Since the vial sensitivity was greater than the digital sensitivity there did not seem to be any point in having the digital level at all or am I missing something?
|Ian P||20/12/2011 11:00:11|
2452 forum posts
I thought the magazine article was a little light in terms of quality content.
In the introduction the author implies that the digital gauge will serve the purpose of a precision level, it obviously will not.
He describes how to turn a test piece to check that the lathe is 'turning accurately' but other than saying it must have the gibs etc adjusted correctly first does not mention that there could at least one other reason why the two bands are not of the same diameter (spindle axis not parallel to the bed?
This whole lathe levelling subject seems to regarded as a sort of holy grail of turning. As I see it, its not so much as levelling the bed but ensuring that it is not twisted. Historically having machines (including lathes) accurately level meant that work and jigs of any shape could be set up by just using a level and squares. Presumably laser devices and other techniques have taken over these days?
One thing I remember reading (in MEW?) that the 'levelling' worriers should think about is, How do you level a lathe on a ship?
|Harold Hall 1||20/12/2011 15:33:08|
|418 forum posts|
Sorry, the link in my post above, whilst working, only works on the bar in front of the word here. Lets try Here again
|Stub Mandrel||20/12/2011 16:11:13|
4311 forum posts
Small lathe users (mini lathe or smaller) may consider not mounting the lathe rigidly. And if anyone think's that's heresy, I'd just point out that the great ETW shared this view. He also pointed out that rigidly mounting the lathe needed great care 'to avoid twisting the bed'
OK a middle sized lathe probably does need to be rigidly mounted for safety (REALLY big ones are pretty immobile, whatever happens!), but is the act of fixing the cause of most of the error that adjustment takes out?
Being a sworn and paid-up pragmatist I'd 100% agree with Tom Walshaw that what really counts are the results of turning, so, in effect measuring tiny angles is, in practice, superfluous as it's just a proxy for the real test - does the lathe turn parallel?
P.S. Douglas - the vial is for setting zero degrees to a decent level of accuracy, you can then use the digital fuinction to set angles, which you can't do with the vial.
PPS. having looked into monolithic accelerometers (as used in levels, Iphones etc.) I woudln't trust them for high levels of accuracy unless you pay a LOT of money. On Farnell's website the prices range from less than £4 to over £400. For about £8 you can get resolution of about 0.1 degree, but that isn't accuiracy and the price jumps up for better resolution.
Edited By Stub Mandrel on 20/12/2011 16:15:53
|903 forum posts|
(REALLY big ones are pretty immobile, whatever happens!)
Well, actually, that isn't the case.
Most really big machines rely in large part on the foundation that they are bolted to for rigidity, as do most quite big ones. Most manufacturers are pretty specific about the foundation requirements (usually specifying concrete density , miniumun thickness & re-enforcement) and the costs of putting in suitable foundations (particularly in areas with poor subsoil) can be considerable.
And failure to do do as suggested can result in poor performance - a large re-built CNC milling machine overhauled by my previous employer was not accurate & could not repeat the alignments achieved in the works when re-installed on site. Before the fitter on site got "stuck in" with a scraper to "sort out" the problem, investigations revealed that the customer had not put the machine back on the prepared foundation it had come off & had inadvertently re-located it across a joint in the floor - the column was on one concrete pad, the base was on another & the two pads were moving suffciently to affect the machine alignment. Re-positioning the machine so that it was all on one pad removed the requirement to "correct" the machine alignment "problem" with a scraper !
It is apparent that many model engineers are not overly interested in the way that industry goes about building & installing machinery, with the "it's good enough" attitude being prevalent. Some, however, may be interested in "trying to do it right" !
|Lol Moore||28/12/2011 18:34:49|
|1 forum posts||
When I read the article I initially thought the same but I think the bubble is used to zero the digital portion so you can then measure angles other than zero using the digital scale - can anyone confirm this?
|Clive Hartland||28/12/2011 22:27:39|
2630 forum posts
Interesting point about the machine bases, some years back we investigated a complaint that our most accurate optical level was inaccurate and unstable!
We carried out tests in our workshop and could find nothing wrong with the level.
Investigation was then transferred to the site of operation which was an assembly jig for a helicopter body.
Arriving in the morning we set up everything and started monitoring as they worked, it seemed the errors were intermittant so we had a long day ahead of us.
We adjourned for dinner and when we returned all the readings were haywire and the jig was out of alignment, every setting was checked again and then we realized that the site was on the bank of a creek and looking out the window showed the tide was in.
The water pushed the sub soil and caused it to heave and later we found out the site was on re-claimed land.
The jig was moved to another location and no more trouble was found.
The intermittant errors were the tide time errors!
The second one was a large concrete plinth that had four wings. It was some 16cub/Mtr of concrete and some 2mtr. deep and after initial drying out we set up the collimators and adjusted them for accuracy.
This was for a specific to task set up for a special sight for use on a Howitzer.
The objective glass was a sphere and had to be glued and clamped exactly.
We had continuous problems with stability until we realized that heavy tanks going past were disturbing the concrete plinth enough to cause severe bubble displacement!
There was no answer except to re-route heavy vehicles another way.
|Clive Hartland||29/12/2011 08:38:31|
2630 forum posts
the only one I have come across was a large cement pedestal that was used as a base for a long distance measuring line, it had a prism that ran along a track over the heads of all the workers and equipment.On advice it was sunk down about a mtr or so but had deeper blocks at each corner.the reason given was that the ground was unstable and the extra pads would stop it moving out of the vertical as we measured to less than a 1/100 of a mm. It was quite an expensive job but we never had a problem.Later the adjoining building had its foundations washed away as it was built on alluvial soil deposited by Glaciers and had 20000tons of soil packed into it to stabilise it.There are a lot of problems with soil stability, particularly as they are now rebuilding over old ground. They have to stabilize the ground by freezing it and then build the casements and they then hold back the soil shift. An example is the light railway and road going into the Dome in London. It started to move so they inserted long rods and froze the ground until they had completed the supports for the rest of it.When you shift thousands of tons of soils something has to move and they monitor all the workings continuously with prisms or reflectors mounted on the construction.This is big business for our company as we supply certified measuring equipment.
|903 forum posts|
The machine was of obscure foreign make but superb quality . Construction was conventional - long fixed bed , travelling table and bridge type milling head slides .
That description could apply to Swiss-made Sip, Dixi or Oerlikon jig borers - fantasic quality and accuracy with prices to match. I could only guess that the bed rose sightly towards the ends as a compensation for the mass of the table & workpiece deflecting the bed slightly during travese.
|martin perman||29/12/2011 13:00:00|
1922 forum posts
With regards to the tides affecting the jig, where I served my apprenticeship was on a large maintenance department of a Lucas CAV manufacturing plant. I went with a fitter to investigate an out of tolerance centering lathe and we could find nothing wrong with it until a forklift went by. When we tapped the concrete pad it was sitting on we got a hollow sound so we brought the forklift back and parked it on the pad near the machine and the bubble shot up one end of its vial. The machine was moved and the whole floor was dug up and relayed.
Our college lecturer used to tell the tale of a Rolls Royce factory where he worked suffering from the trains over a mile away causing vibrations to appear on turning work.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.